In compressed air systems it is common practice to pass the pressurized air through a filter before it is delivered to an air motor or other place of use. The filter not only removes solid impurities but will also collect water or other liquids entrained in the compressed air. This water or other liquid must be periodically drained from the filter bowl.
To remove the liquid, the filter bowls are frequently provided with a normally closed manually operated drain valve. In one prior form of such drain valve for a filter bowl as shown in U.S. Pat. No. 3,450,146, the valve body is of flexible material and has a tubular extension below the bowl that may be manually bent for tilting a valve element for unseating the latter. However, the valve stem carries a secondary seal that only opens when the system is pressurized and the body is manually bent. Thus, the bowl cannot be readily drained when it is not under pressure.